[foreign-memaccess+abi] RFR: 8315769: Add support for sliced allocation [v3]

[Jorn Vernee]

On Wed, 6 Sep 2023 15:04:02 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Maurizio Cimadamore has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Add `throws` clause re. hyper-aligned layouts
>>  - Address review comments
>
> src/java.base/share/classes/java/lang/foreign/SegmentAllocator.java line 324:
> 
>> 322:      * @param elements the short elements to be copied to the newly allocated memory block.
>> 323:      * @return a segment for the newly allocated memory block.
>> 324:      * @throws IllegalArgumentException if {@code elementLayout.byteAlignment() > elementLayout.byteSize()}.
> 
> I've added this to all the array-accepting allocateFrom, as I think this will be thrown by the underlying copy op. There are a lot of other exceptions that memory copy could throw, in the following conditions:
> 1. layout size * element count is too big
> 2. offset/elementCount < 0
> 3. the interval (offset, size) falls outside the spatial bounds of the segment
> 4. access occurs from a thread other than the thread owning segment
> 5. access occurs when segment scope is no longer alive
> 6. start of segment is not aligned according to provided layout
> 
> Now, I think 1-5 are impossible because:
> 1. number of bytes is always derived from an array size, no overflow is possible
> 2. default impl guarantees positive lengths/offsets
> 3. access is provably in bounds (from 0 to array size)
> 4. array segment can be accessed from any thread
> 5. array segment is always alive
> 
> As for (6), I think right now it can happen, but I think we should probably rectify the implementation of the array-accepting `allocateFrom` to pass `JAVA_SHORT`, `JAVA_CHAR`, ... as the "source layout element" rather than just passing a modified version of the input destination layout. If we do that, then access would be aligned by construction.

Okay, I agree with your last statement. We seem to be on the same page there.

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PR Review Comment: https://git.openjdk.org/panama-foreign/pull/878#discussion_r1317619001